Study of the asphaltene aggregation using the NIR in combination with principal component analysis and dynamic model development

The Near-infrared spectroscopy (NIR) has been used in many studies with the aim to obtain the best knowledge of the phenomena associated to behavior of mixture petroleum and petroleum. Besides, this technique had been used as tool to expect physical and chemical properties of the molecules, for instance, the SARA (Saturate, Aromatic, Resin and Asphaltenic) analysis, total acid number, interfacial elasticity, interfacial tension, molecular weight, viscosity, size of particles and others.
In addition to molecular absorption, the NIR spectrum is dependent upon several physical parameters, where the most prominent is scattering from particles. When the particle size increases, the amount of light scattered by the sample is changed. One of the main advantages of near infrared spectroscopy when working with colloidal systems like crude oils is the ability to also gain information on the physical state of the system. The NIR will display a baseline elevation due to light scattering by aggregates or particles in solution.
The NIR in combination with principal component analysis (PCA) is shown to be an efficient tool in detecting both bubble points and asphaltene aggregation onset pressures in high-pressure systems. By use of principal component analysis (PCA) the effect of asphaltene aggregation is easily distinguished from the effect of fluid compressibility, an effect which also influences the high-pressure NIR spectra.
The studies of this work were conducted to the NIR analysis of the petroleum with different characteristics relating to composition (SARA) and its mixtures, aiming to obtain a dynamic model able to identify the samples that show a disposition to promote the asphaltenes aggregation phenomena and to promote the emulsion formation. The dynamic models applied in this work were the first order pure, the first order of inverse response and second order pure.
Crude oil samples at ambient temperature and pressure were received from exploration sites located in West Africa and Brazil.These crude oils were chosen to form a sample set of oils differing in physical properties. The first step of data treatment was to select the appropriate region of NIR for this application (11,000 – 4000 cm-1) shows the optical absorption (optical density) vs. wavelength for many crude oils for a 10 mm pathlenght. The scan of peaks corresponds to overtone bands of primarily saturated CH2 and CH3 groups. These peak energies are the 7200 cm-1 (two-stretch+bend) and the 8400 cm-1 (three-stretch).The broad, increasing absorption profile corresponds to the absorption edge of electronic excitation. Previous work has shown that, even in the NIR, absorption rather than scattering from dispersed (not flocculated) asphaltene particles produces this optical density of crude oils. Of course, if the crude oil has an insoluble was phase or flocculated oil will be highly altered. For crude oils, the wavelength range of increasing absorption is much broader than the absorption bands of individual molecules and is due to the overlapping spectra of many molecular components. The spectral location of the absorption edge varies considerably and almost continuously for the different crude oils.